Elevators consist in essence of a load-carrying means and a compensating weight, which are connected together by a suspension means. The suspension means is usually formed of a plurality of suspension ropes (steel ropes); there are, however, also synthetic-fiber ropes, flat belts, lengthwise-ribbed belts, and similar. The load-carrying means is (particularly in the case of passenger elevators) usually an elevator car.
Elevators, particularly their load-carrying means, must be equipped with safety devices. Should a specified speed in a downward direction be exceeded, the safety device triggers and brakes the elevator. Such safety devices that act in the downward direction are particularly intended for the case of a breakage of the suspension means. In this case, the elevator accelerates downward at 1 g (where g is the acceleration due to gravity). The safety devices must therefore generate a substantially greater force than the weight force (weight force being the mass multiplied by the acceleration due to gravity), since otherwise the elevator would not decelerate. Similar considerations apply to the case where the compensating weight is equipped with a safety device.
High-speed elevators must also be equipped with a safety device that also acts in the case of upward travel, which triggers at excessively fast upward travel. These safety devices that act in the case of upward travel are activated in, for example, the third-highest floor and brake the elevator, should the control system fail and the upward travel not be decelerated in good time before reaching the highest floor. Safety devices that act in the case of upward travel must decelerate considerably less slowly, since, in the case of deceleration greater than the acceleration due to gravity, the momentum of the passengers would cause them to be thrown against the ceiling of the elevator. The forces that must be generated by the safety device in the upward direction are therefore correspondingly smaller than the forces that must be generated for downward travel.
The safety devices must be self-locking: once they have engaged, they hold the elevator until they are released. Many safety devices are designed so that they can be released by the elevator, or its load-carrying means, being moved counter to the direction of engagement, since this does not require a separate mechanism to release the safety devices. That is to say, if the safety device engaged while on a downward travel or an upward travel, the elevator is moved either with the drive apparatus, or with an auxiliary device, counter to the direction of travel, as a result of which the safety device releases itself.
Problematical in this case is that the releasing forces may be very large, so that the drive apparatus cannot generate these high forces. The forces to release the safety device are particularly high if the safety device triggered during downward travel, since in this case the braking forces are also correspondingly large.
To solve this problem, in the generic EP 1213247 A1 the proposal is made that the load-carrying means has a certain play relative to the safety device. On engagement of the safety device, the drive unit can therefore first accelerate the load-carrying unit for a few centimeters before the load-carrying means strikes the safety device and triggers the latter, whereby not only the force of the drive unit acts but also the much greater momentum force of the elevator. The safety device is hence struck free like with a hammer.
In this method it is necessary for the safety device to be designed in such manner that when the safety device is in engagement, the load-carrying means can be slightly moved. Without such a safety device, this method cannot be applied. This method therefore requires a mechanical modification of the elevator.
Furthermore, from EP 1641700 B1 a method for releasing a load-carrying means from engagement has become known in which a wire rope is fastened to the load-carrying means and to the compensating weight, which, at the bottom of the hoistway, is reversed around a free-running pulley which is loaded with a tension weight. Depending on whether the safety device engaged during upward travel or downward travel, the wire rope is pulled in one or other direction by means of a chain hoist. In this method, the safety device can be embodied arbitrarily: it is not necessary for the safety device in the engaged position to allow a limited movement of the load-carrying means. However, a disadvantage of this method is that, to release the load-carrying means from engagement, an additional chain hoist with corresponding drive must be fastened to the wire rope.